JP6984185B2 - Positive electrode active material, positive electrode and sodium ion battery, and method for manufacturing positive electrode active material - Google Patents

Description

The present invention relates to a positive electrode active material, a positive electrode and a sodium ion battery, and a method for producing the positive electrode active material, and in particular, a positive electrode active material, a positive electrode and a sodium ion battery used in a sodium ion battery such as a sodium ion secondary battery. In addition, the present invention relates to a method for producing a positive electrode active material.

Conventionally, in a sodium ion battery such as a sodium ion secondary battery, it has been studied to use a layered metal oxide as a positive electrode active material (see Patent Document 1).

Japanese Patent No. 5870930

By the way, when a layered metal oxide is used as the positive electrode active material for a sodium ion battery, the crystal structure is unstable due to the weak bond between the metal atom and the oxygen atom, and the deterioration due to charging / discharging may be accelerated. There is sex.

Therefore, an object of the present invention is a positive electrode active material for a sodium ion battery having a stable crystal structure capable of further improving the battery performance of the sodium ion battery, a positive electrode and a sodium ion battery, and a method for producing the positive electrode active material. Is to provide.

The positive electrode active material according to the present invention is a positive electrode active material for a sodium ion battery, and is Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, It is characterized by containing an iron phosphate compound represented by 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1).

In the positive electrode active material according to the present invention, the iron phosphate compound is characterized by satisfying c + d = 1.

In the positive electrode active material according to the present invention, the iron phosphate compound is characterized by satisfying b-1 = c / 3 = (1-d) / 3.

Positive electrode active material according to the present invention is a positive electrode active material for sodium ion _battery, includes iron phosphate compounds represented by _{Na a Fe b (PO 4)} (OH) c (H 2 O) d, The iron phosphate compound has the highest intensity first peak in the range of 2θ = 26.05 ° or more and 26.95 ° or less in XRD measurement using CuKα ray as an X-ray source, and 2θ = 27.15. It is characterized by having a second peak with the second highest intensity in the range of ° or more and 28.20 ° or less.

In the positive electrode active material according to the present invention, the crystal structure of the iron phosphate compound is characterized in that it belongs to the _{space group I4 1 / amd.}

The positive electrode according to the present invention is characterized by containing the above-mentioned positive electrode active material.

The sodium ion battery according to the present invention is characterized by including the above-mentioned positive electrode.

The method for producing a positive electrode active material according to the present invention is a method for producing a positive electrode active material for a sodium ion battery, which is a raw material solution preparation step of dissolving an iron raw material and a phosphorus raw material in water to prepare a raw material solution. It is characterized by comprising a hydrothermal synthesis step of heat-treating the raw material solution at 150 ° C. or higher and lower than 300 ° C. for hydrothermal synthesis.

The method of manufacturing a positive electrode active material according to the present invention, wherein the iron raw _{material, Fe (NO 3) 3 ·} 9H 2 O, FeC 2 O 4 · 2H 2 O, Fe (NH 4) 2 (SO 4) 2 · 4H _It contains at least one of 2 O, and the phosphorus raw material is characterized by containing _{H 3} PO _4.

In the method for producing a positive electrode active material according to the present invention, the raw material solution preparing step is characterized in that the raw material solution contains a first sodium raw material.

In the method for producing a positive electrode active material according to the present invention, the first sodium raw materials are NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄・ 12H ₂ O and Na _2. It is characterized by containing at least one of HPO _4.

In the method for producing a positive electrode active material according to the present invention, a compound hydrothermally synthesized in the hydrothermal synthesis step and a second sodium raw material are mixed to form a mixture, and the mixture is heated at 150 ° C. or higher. It is characterized in that it is heat-treated at 500 ° C. or lower to cause a solid-phase reaction.

In the method for producing a positive electrode active material according to the present invention, the second sodium raw material is NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄・ 12H ₂ O and Na _2. It is characterized by containing at least one of HPO _4.

The method for producing a positive electrode active material according to the present invention is characterized in that the mixture is heat-treated in a reducing atmosphere to cause a solid-phase reaction.

In the method for producing a positive electrode active material according to the present invention, a mixed solution is prepared by dissolving a compound hydrothermally synthesized in the hydrothermal synthesis step and a third sodium raw material in water, and the mixed solution is prepared as 150. It is characterized by hydrothermal synthesis by heat treatment at ° C. or higher and lower than 300 ° C.

In the method for producing a positive electrode active material according to the present invention, the third sodium raw material is NaNO ₃ , Na ₂ CO ₃ , NaOH, CH ₃ COONa, NaH ₂ PO ₄ , Na ₃ PO ₄・ 12H ₂ O and Na _2. It is characterized by containing at least one of HPO _4.

The method for producing a positive electrode active material according to the present invention is characterized in that the mixed solution is heat-treated in a reducing atmosphere for hydrothermal synthesis.

In the method for producing a positive electrode active material according to the present invention, a battery is assembled using a synthetic product hydrothermally synthesized in the hydrothermal synthesis step as a positive electrode active material and a material containing sodium as a negative electrode active material to cause a discharge reaction. The compound is electrochemically characterized by containing sodium.

According to the above configuration, the movement of sodium ions in the crystal is easy and the crystal structure is stable, so that it is possible to improve the battery performance of the sodium ion battery.

In the embodiment of the present invention, Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1). It is a flowchart which shows the process of producing IHP which is the iron phosphate compound represented. It is a graph which shows the X-ray diffraction measurement result of the IHP of Example 3 in the Embodiment of this invention. It is a figure which shows the crystal structure analysis result of the IHP of Example 3 in the Embodiment of this invention. It is a figure which shows the crystal structure analysis result of the IHP of Examples 1 to 7 in the Embodiment of this invention. It is a figure for demonstrating the reaction which occurs in the positive electrode and the negative electrode of a sodium ion battery in embodiment of this invention. It is a figure which shows the Na amount of IHPNa of Examples 8 to 25 in the Embodiment of this invention. It is a figure which shows the Na amount of IHPNa of Examples 27-34 in embodiment of this invention. In the embodiment of the present invention, it is a graph which shows the charge / discharge characteristic of the coin cell using the positive electrode active material which consists of IHP. In the embodiment of the present invention, it is a graph which shows the charge / discharge characteristic of the coin cell using the positive electrode active material which consists of IHPNa. It is a graph which shows the charge / discharge cycle characteristic in embodiment of this invention. In the embodiment of the present invention, it is a graph which shows the X-ray diffraction measurement result of the positive electrode active material in each discharge state.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Sodium-ion batteries such as sodium-ion secondary batteries are used in, for example, notebook computers, mobile phones, electric vehicles, and the like.

Positive electrode active material for a sodium ion _{battery, Na a Fe b (PO 4} ) (OH) c (H 2 O) d ( where, 0 ≦ a ≦ 1.333,1.000 ≦ b ≦ 1.333,0 It contains an iron phosphate compound represented by ≦ c ≦ 1 and 0 ≦ d ≦ 1). This iron phosphate compound preferably satisfies c + d = 1. Further, it is more preferable that this iron phosphate compound satisfies b-1 = c / 3 = (1-d) / 3.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (However, 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1 The crystal structure of the iron phosphate compound represented by) is composed of tetragonal crystals. The crystal structure of this iron phosphate compound is preferably a crystal structure belonging to _{I4 1} / amd of the space group represented by the Hermann-Morgan symbol (international symbol). In the crystal structure of this iron phosphate compound, sodium ions can easily move in the crystal, and sodium ions can be easily reversibly deinserted and inserted, and can function as a positive electrode active material of a sodium ion battery. .. Further, since this iron phosphate compound _{contains an oxygen atom in the form of PO 4} , the PO bond becomes strong and the structural stability becomes high. If the numerical range of a, b, c, and d is not satisfied, the crystal structure may change and the phase may change to an electrochemically less active phase.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (However, 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1 The iron phosphate compound represented by) has the highest intensity in the range of 2θ = 26.05 ° or more and 26.95 ° or less in XRD measurement (X-ray diffraction measurement) using CuKα ray as an X-ray source. It has one peak and a second peak with the second highest intensity in the range of 2θ = 27.15 ° or more and 28.20 ° or less.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (However, 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1 The iron phosphate compound represented by) is Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1. It is preferable that the iron phosphate compound is represented by 333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) (hereinafter, may be referred to as IHP). The crystal structure of IHP is a tetragonal crystal, and it is preferable that the crystal structure belongs to _{I4 1} / amd of the space group represented by the Hermann-Morgan symbol (international symbol).

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (However, 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1 _{The iron phosphate compound represented by) is Na a} Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 <a ≦ 1.333) when a is not 0 (a ≠ 0). , 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) may be an iron phosphate compound (hereinafter, may be referred to as IHPNa). The crystal structure of IHPNa is a tetragonal crystal, and it is preferable that the crystal structure belongs to _{I4 1} / amd of the space group represented by the Hermmann-Morgan symbol (international symbol).

Next, a method for producing a positive electrode active material for a sodium ion battery will be described. The method for producing the positive electrode active material for a sodium ion battery is as follows: Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (However, 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1. It includes a step of producing an iron phosphate compound represented by 333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1).

First, _{iron phosphate represented by Fe b} (PO ₄ ) (OH) _c (H ₂ O) _d (however, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1). A method for producing IHP, which is a compound, will be described. FIG. 1 shows _{phosphorus represented by Fe b} (PO ₄ ) (OH) _c (H ₂ O) _d (where 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1). It is a flowchart which shows the process of producing IHP which is an iron acid compound. The step of producing IHP includes a raw material solution preparation step (S10) and a hydrothermal synthesis step (S12).

The raw material solution preparation step (S10) is a step of dissolving an iron raw material and a phosphorus raw material in water to prepare a raw material solution.

Iron raw material, for _{example, Fe (NO 3) 3 ·} 9H 2 O, FeC 2 O 4 · 2H 2 O and _{Fe (NH 4) 2 (SO} 4) 2 · 2H 2 iron compound containing at least one of O Is possible, but is not particularly limited to these iron compounds. As the iron raw material, these iron compounds may be used alone or in combination of two or more. As these iron compounds, general commercial products can be used.

As the phosphorus raw material, for example, _{a phosphorus compound such as H 3} PO ₄ can be used, but the phosphorus compound is not particularly limited. As the phosphorus compound, a general commercially available product can be used.

The iron raw material and the phosphorus raw material are dissolved in water such as deionized water (ion-exchanged water) to prepare a raw material solution.

The hydrothermal synthesis step (S12) is a step of hydrothermally synthesizing the prepared raw material solution by heat-treating it at 150 ° C. or higher and lower than 300 ° C.

The prepared raw material solution is, for example, sealed in a pressure-resistant sealed container such as a carbon steel vessel, heated and pressurized by an induction heating furnace or the like, and heat-treated to be hydrothermally synthesized. Since the raw material solution is sealed in a pressure-resistant sealed container, when heated in an induction heating furnace or the like, it is heated at a high pressure and hydrothermally synthesized.

When the raw material solution is sealed in a pressure-resistant sealed container, it is preferable to spray an inert gas such as argon gas to replace the air. The atmosphere inside the pressure-resistant sealed container may be an air atmosphere, but may be an inert gas atmosphere such as argon gas. The heating furnace is not particularly limited as long as it can be heated at 150 ° C. or higher and lower than 300 ° C., but a swing type induction heating furnace or a general muffle furnace can be used.

The heat treatment temperature is preferably 150 ° C. or higher and lower than 300 ° C. This is because when the heat treatment temperature is lower than 150 ° C. or when the heat treatment temperature is higher than 300 ° C., it becomes difficult to generate IHP. The pressure in the pressure-resistant sealed container is preferably 2 MPa or more and 50 MPa or less.

The heat treatment time is preferably 10 minutes to 4 hours. This is because if the heat treatment time is shorter than 10 minutes, the hydrothermal synthesis reaction is not completed, so that the yield of IHP may decrease. This is because if the heat treatment time is longer than 4 hours, the performance of the IHP may deteriorate and the performance of the sodium ion battery may deteriorate.

The product produced by hydrothermal synthesis is separated from the residual solution by a centrifuge or the like. The product is washed with deionized water (ion-exchanged water) or the like, and then dried by a dryer or the like at, for example, 30 ° C. or higher and 150 ° C. or lower. Further, the product may be vacuum-dried after being washed with deionized water (ion-exchanged water) or the like. In this way, IHP is produced in the form of powder or the like.

Next, Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 <a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ A method for producing IHPNa, which is an iron phosphate compound represented by d ≦ 1), will be described. The IHPNa can be generated by four kinds of generation methods. More specifically, the method for producing IHPNa includes a method for directly synthesizing IHPNa, a method for producing it by a solid-phase reaction method using IHP, and a method for producing it by a hydrothermal synthesis method using IHP. There is a method of producing by an electrochemical method using IHP.

First, a method of directly synthesizing and generating IHPNa will be described. The method for directly synthesizing and producing IHPNa is different from the above-mentioned method for producing IHP in the raw material solution preparation step. In the raw material solution preparation step, the first sodium raw material, the iron raw material, and the phosphorus raw material are dissolved in water to prepare a raw material solution.

The first sodium raw material, for _{example, NaNO 3, Na 2 CO 3} , NaOH, CH 3 COONa, NaH 2 PO 4, Na 3 sodium compound containing at least one of PO 4 · 12H ₂ O and _Na 2 HPO ₄ Is possible, but is not particularly limited to these sodium compounds. As the first sodium raw material, these sodium compounds may be used alone or in combination of two or more. As the first sodium raw material, a general commercial product can be used. As the iron raw material and the phosphorus raw material, the same raw materials as the iron raw material and the phosphorus raw material used in the raw material solution preparation step (S10) can be used.

The first sodium raw material, iron raw material, and phosphorus raw material are dissolved in water such as deionized water (ion-exchanged water) to prepare a raw material solution. The first sodium raw material, the iron raw material, and the phosphorus raw material may be mixed so that Na is 2 times or more and 30 times or less in terms of the substance amount ratio with Fe. Further, it is preferable that P is mixed so as to have a substance amount ratio of 1 time or more and 10 times or less with Fe. For example, Na, Fe, and P are preferably Na: Fe: P = 10: 1: 1 in terms of the amount of substance ratio of each element. The pH of the raw material solution may be pH 4 or less.

The prepared raw material solution is heat-treated at 150 ° C. or higher and lower than 300 ° C. for hydrothermal synthesis. As the hydrothermal synthesis may be carried out by the same method as the above-mentioned hydrothermal synthesis step (S12), detailed description thereof will be omitted. In this way, IHPNa is produced in the form of powder or the like.

Next, a method for producing by a solid-phase reaction method using IHP will be described. First, the powdered IHP and the second sodium raw material are mixed to form a mixture. The powdery IHP can be produced by performing the above-mentioned raw material solution preparation step (S10) and the hydrothermal synthesis step (S12). The second sodium raw material, for _{example, NaNO 3, Na 2 CO 3} , NaOH, CH 3 COONa, NaH 2 PO 4, Na 3 sodium compound containing at least one of PO 4 · 12H ₂ O and _Na 2 HPO ₄ Is possible, but is not particularly limited to these sodium compounds. As the second sodium raw material, these sodium compounds may be used alone or in combination of two or more. The powdered IHP and the second sodium raw material are mixed to form a pellet-like mixture.

Next, the mixture is heat-treated at 200 ° C. or higher and 500 ° C. or lower for a solid-phase reaction. The heat treatment temperature is preferably 200 ° C. or higher and 500 ° C. or lower. This is because when the heat treatment temperature is lower than 200 ° C. or when the heat treatment temperature is higher than 500 ° C., it becomes difficult to generate IHPNa.

The heat treatment time is preferably 10 minutes to 4 hours. This is because if the heat treatment time is shorter than 10 minutes, the solid phase reaction is not completed and the yield of IHPNa may decrease. This is because if the heat treatment time is longer than 4 hours, the performance of IHPNa is deteriorated, so that the performance of the sodium ion battery may be deteriorated.

The mixture may be heat-treated in an inert atmosphere such as argon gas or a reducing atmosphere containing hydrogen gas or the like. The mixture is preferably heat treated in a reducing atmosphere in order to insert more Na into the IHP. The heating furnace is not particularly limited as long as it can be heated at 200 ° C. or higher and 500 ° C. or lower, but a swing type induction heating furnace or a general muffle furnace can be used. In this way, IHPNa is produced in the form of powder or the like.

Next, a method of producing by a hydrothermal synthesis method using IHP will be described. First, powdered IHP and a third sodium raw material are dissolved in ion-exchanged water to prepare a mixed solution. The powdery IHP can be produced by performing the above-mentioned raw material solution preparation step (S10) and the hydrothermal synthesis step (S12). The third sodium raw material, for _{example, NaNO 3, Na 2 CO 3} , NaOH, CH 3 COONa, NaH 2 PO 4, Na 3 sodium compound containing at least one of PO 4 · 12H ₂ O and _Na 2 HPO ₄ Is possible, but is not particularly limited to these sodium compounds. As the third sodium raw material, these sodium compounds may be used alone or in combination of two or more. A mixed solution is prepared by dissolving powdered IHP and a third sodium raw material in water such as deionized water (ion-exchanged water).

Next, the mixed solution is heat-treated at 150 ° C. or higher and lower than 300 ° C. for hydrothermal synthesis. The prepared mixed solution is enclosed in a pressure-resistant sealed container such as a carbon steel vessel together with a reducing agent, and is hydrothermally synthesized by heating and pressurizing in a reducing atmosphere with an induction heating furnace or the like. By heating and pressurizing in a reducing atmosphere, Fe is reduced, Na is inserted, and IHPNa is produced. Ascorbic acid can be used as the reducing agent, for example, but other reducing agents may be used. In this way, IHPNa is produced in the form of powder or the like.

Next, a method of producing by an electrochemical method using IHP will be described. IHP can be produced by performing the above-mentioned raw material solution preparation step (S10) and hydrothermal synthesis step (S12). IHP is used as a positive electrode active material, and a material containing sodium is used as a negative electrode active material. By assembling a battery and causing a discharge reaction, sodium is electrochemically contained in IHP. As the material containing sodium as the negative electrode active material, metallic sodium or the like can be used, but the material is not particularly limited. This causes an insertion reaction of sodium into IHP to produce IHPNa.

Next, a positive electrode for a sodium ion battery will be described.

The positive electrode is Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ It includes a positive electrode active material containing an iron phosphate compound represented by d ≦ 1), a conductive material, a binder, a thickener, and a current collector. IHP may be used or IHPNa may be used for this iron phosphate compound.

As the conductive material, Ketjen black, acetylene black, graphite, carbon fiber and the like can be used. As the binder, a fluororesin such as styrene-butadiene rubber and polytetrafluoroethylene, a polyethylene resin, a polypropylene resin and the like can be used. Carboxymethyl cellulose or the like can be used as the thickener. As the current collector, a metal foil or a metal sheet made of aluminum, stainless steel, copper, nickel or the like can be used.

Next, a method for manufacturing the positive electrode will be described. First, Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d A slurry is prepared by mixing a positive electrode active material containing an iron phosphate compound represented by ≦ 1), a conductive material, a binder, and a thickener. Regarding the mixing ratio of the positive electrode active material, the conductive material, the binder, and the thickener, for example, in terms of mass ratio, the positive electrode active material: conductive material: binder: thickener = 80: 10: 6: 4. It is good to say. As a mixing method of these materials, a general mixing method using a ball mill or the like can be used.

The produced slurry is applied to the surface of a current collector such as a metal foil. The slurry can be applied to the current collector by an application method using a general brush, spatula, or the like. The current collector coated with the slurry is dried by a dryer or the like at, for example, 30 ° C. or higher and 150 ° C. or lower. In this way, a positive electrode for a sodium ion battery is manufactured.

Next, the sodium ion battery will be described. As an example, a sodium ion secondary battery will be described.

The sodium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode is Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ It includes a positive electrode active material containing an iron phosphate compound represented by d ≦ 1). IHP may be used or IHPNa may be used for this iron phosphate compound. The negative electrode is, for example, TiO ₂ , Nb-doped TiO ₂ , lithium titanate (Li ₄ Ti ₅ O ₁₂ ), Sn, Si, SiO, SnO, Sn—Cu alloy thin film, Sn—Fe alloy thin film, carbon nanotube, Si−. It is composed of Sn—Ti alloy, carbon nanofibers, MoO ₃ , SnO ₂ , P alone, metallic sodium, hard carbon, graphite, and Pb. When the positive electrode is made of a positive electrode active material containing IHP, the negative electrode may be made of a material containing sodium such as metallic sodium. The electrolyte is composed of, for example, _{a mixture of solute 1M NaClO 4} / solvent propylene carbonate and 2% by volume fluoroethylene carbonate. The separator is made of, for example, a non-woven fabric made of glass fiber or the like. The shape of the sodium-ion secondary battery can be a coin type, a cylindrical type, a square type, or the like.

As described above, according to the above configuration, the positive electrode active material for a sodium ion _{battery, Na a Fe b (PO 4} ) (OH) c (H 2 O) d ( where, 0 ≦ a ≦ 1.333,1.000 Since the iron phosphate compound represented by ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) is contained, the battery performance of the sodium ion battery can be improved. _{This iron phosphate compound has a stable crystal structure belonging to I4 1} / amd of the space group represented by the Hermann-Morgan symbol (international symbol), sodium ions easily move in the crystal, and sodium. Ions can be easily reversibly removed and inserted, and can function as a positive electrode active material for sodium-ion batteries.

Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (However, 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1. The iron phosphate compound represented by 333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1) was produced.

(Generation of INP)
First, _{iron phosphate represented by Fe b} (PO ₄ ) (OH) _c (H ₂ O) _d (however, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ d ≦ 1). IHP, which is a compound, was produced. The method of generating IHP of Examples 1 to 7 will be described.

An iron raw material and a phosphorus raw material were dissolved in ion-exchanged water to prepare a raw material solution. Fe (NO ₃ ) and 9H ₂ O (purity 99%) were used as the iron raw material. _{H 3} PO ₄ (purity 85%) was used as the phosphorus raw material. The concentration of the iron raw material was 0.1 mol / l. In the IHPs of Examples 1 to 7, the heat treatment conditions were changed.

The raw material solution was placed in a pressure-resistant sealed container and sealed while replacing air by spraying argon gas. A pressure-resistant sealed container containing the raw material solution was heated in a swing-type induction heating furnace for hydrothermal synthesis. The heating temperature was 150 ° C. or higher and lower than 300 ° C. The heating time was 10 minutes to 4 hours. After the synthesis, only the compound was extracted by a centrifuge. The extracted compound was vacuum dried to obtain a powdery IHP.

The generated IHPs of Examples 1 to 7 were subjected to crystal structure analysis by an X-ray diffraction measurement method. Rigaku's fully automatic horizontal multipurpose X-ray diffractometer SmartLab is used for X-ray diffraction measurement. The X-ray source is Cu, the acceleration voltage is 45 kV, the acceleration current is 200 mA, and the step angle is 0.02 degrees. The folding angle 2θ was measured under the condition of 10 ° to 90 °. Crystal structure analysis was performed from the measured data using the RIETAN-FP program and the Rietveld method.

As a representative, the crystal structure analysis method of IHP of Example 3 will be described. FIG. 2 is a graph showing the results of X-ray diffraction measurement of IHP of Example 3. In the graph of FIG. 2, the diffraction angle 2θ is taken on the horizontal axis, the intensity is taken on the vertical axis, and the measurement data and the analysis result are shown by a solid line. _{Incidentally, R wp} is 7.670, _{R e} is 4.787, S is 1.6023. Crystal structure analysis was performed from this measurement data using the Rietveld method. FIG. 3 is a diagram showing the results of crystal structure analysis of IHP of Example 3. FIG. 3 shows the types of elements constituting the IHP of Example 3, the occupancy rate of each site of each element, and the x, y, and z coordinates of each element. The chemical composition of IHP of Example 3 is b = 1.135, c = 0.405, d = 0.595. Fe _1.135 (PO ₄ ) (OH) _0.405 (H ₂ O) _0. It turned out to be _595. Further, the crystal structure of IHP of Example 3 was tetragonal and belonged to the _{space group I4 1 / amd represented by the Hermann-Morgan symbol (international symbol).}

For the IHPs of other examples, crystal structure analysis was performed in the same manner as the IHPs of Example 3. FIG. 4 is a diagram showing the crystal structure analysis results of IHP of Examples 1 to 7. In Fe 1 + x (PO ₄ ) (OH) _3x (H ₂ O) _1-3x , which is the IHP theoretical formula, _{1 + x of} Fe amount corresponds to b, 3x of OH amount corresponds to c, and H ₂ O amount. 1-3x corresponds to d. _{Further, the values of Fe, PO 4} , O, and H in IHP of each example indicate the amount of each element or the like. For example, in the case of IHP in Example 1, Fe amount is 1.110, PO ₄ weight 1.000, O weight 5.000, H amount indicates that the 1.671. Further, the value of X in IHP of each embodiment corresponds to x in the IHP theoretical formula. In the chemical composition of IHP of Examples 1 to 7, the Fe amount is 1.000 ≦ b ≦ 1.333, the OH amount is 0 ≦ c ≦ 1, and the H ₂ O amount is 0 ≦ d ≦ 1. rice field. Further, the crystal structures of IHPs of Examples 1 to 7 were tetragonal and belonged to the _{space group I4 1 / amd represented by the Hermann-Morgan symbol (international symbol).}

(Generation of INPNa)
Next, Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (where 0 <a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 ≦ c ≦ 1, 0 ≦ IHPNa, which is an iron phosphate compound represented by d ≦ 1), was produced. The method for producing IHPNa was carried out by three types of methods: solid phase reaction method, hydrothermal synthesis method and electrochemical method.

First, a method for producing a solid-phase reaction method will be described. IHPNa of Examples 8 to 25 was produced by the solid phase reaction method. The powdered IHP and the sodium raw material were mixed to form a mixture. In Examples 8 to 25, the IHP of Example 3 was used as the powdered IHP. In Examples 8 to 23, NaOH was used as a sodium raw material. In Example 24, Na ₂ CO ₃ was used as a sodium raw material. In Example 25, NaNO ₃ was used as a sodium raw material.

The mixture was subjected to a solid phase reaction by heating in an argon gas atmosphere or a mixed gas atmosphere of hydrogen gas and argon gas. The concentration of hydrogen gas in the mixed gas was 5% by mass. The synthesis temperature was 200 ° C to 500 ° C. The synthesis time was 2 to 108 hours. In this way, IHPNa of Examples 8 to 25 was obtained.

Next, for IHPNa of Examples 8 to 25, the amount of Na was measured from the battery capacity. First, a method of obtaining the amount of Na from the battery capacity will be described. FIG. 5 is a diagram for explaining the reaction occurring between the positive electrode and the negative electrode of the sodium ion battery. Sodium ion Na ⁺ diffuses and moves inside the battery. The electron e ⁻ travels through a circuit outside the battery and becomes an electric current. From this, when the current value (charge amount) is measured, ^{the amount of sodium ion Na +} transferred inside the battery can be obtained.

More specifically, the number of moles ^{of the transferred electrons e −} can be obtained by dividing the amount of charge (As) measured by charging and discharging the battery by the Faraday constant (As / mol). In the battery, the sodium ion Na ⁺ moves in the same amount as the electrons, so the calculated ^{number of moles of the electron e −} becomes the number of moles of the ^{sodium ion Na +} that has moved as it is. Since the number of moles of the material used for the electrode is determined from the weight of the electrode, the amount of Na in IHPNa can be determined.

FIG. 6 is a diagram showing the amount of Na in IHPNa of Examples 8 to 25. In addition, in FIG. 6, the amount of Na is shown together with the synthesis conditions of Examples 8 to 25. In IHPNa of Examples 8 to 25, it was found that the amount of Na was in the range of 0 <a≤1.333. Further, as a result of crystal structure analysis of IHPNa of Examples 8 to 25 by the X-ray diffraction measurement method in the same manner as the crystal structure analysis of IHP of Examples 1 to 7, the crystals of IHPNa of Examples 8 to 25 were obtained. The structure was tetragonal and belonged to the _{space group I4 1} / amd represented by the Hermann-Morgan symbol (international symbol).

Next, a method for producing IHPNa by a hydrothermal synthesis method will be described. The IHPNa of Example 26 was produced by a hydrothermal synthesis method. The powdered IHP and the sodium raw material were dissolved in ion-exchanged water to prepare a mixed solution. As the powdered IHP, the IHP of Example 3 was used. _{NaNO 3} was used as the sodium raw material.

The mixed solution and the reducing agent were placed in a pressure-resistant sealed container, and the mixture was sealed while replacing air by spraying argon gas. Ascorbic acid was used as the reducing agent. A pressure-resistant sealed container containing a mixed solution and a reducing agent was heated in a swing-type induction heating furnace for hydrothermal synthesis. The synthesis temperature was 150 ° C. The synthesis time was 20 minutes. After the synthesis, only the compound was extracted by a centrifuge. The extracted compound was vacuum dried to obtain powdered IHPNa.

Next, with respect to IHPNa of Example 26, the amount of Na was measured from the battery capacity by the same method as IHPNa of Examples 8 to 25. The Na amount of IHPNa in Example 26 was 0.090, and the battery capacity was 12 (mAh / g). From this measurement result, it was found that the amount of Na in IHPNa of Example 26 was in the range of 0 <a≤1.333. Further, as a result of performing crystal structure analysis of IHPNa of Example 26 by an X-ray diffraction measurement method in the same manner as the crystal structure analysis of IHP of Examples 1 to 7, the crystal structure of IHPNa of Example 26 is tetragonal. It is a crystal and belongs to the _{space group I4 1} / amd represented by the Hermann-Morgan symbol (international symbol).

Next, a production method by an electrochemical method will be described. The electrochemical method produced IHPNa from Examples 27 to 34. A battery was assembled using powdered IHP as a positive electrode active material and sodium as a negative electrode active material to cause a discharge reaction. The battery type was a coin cell. Then, by causing a discharge reaction, sodium was inserted into IHP to obtain IHPNa.

For Examples 27 to 30, the IHP of Example 3 was used. In Examples 31 to 34, for the method for synthesizing the IHP of Example 3, IHP synthesized by heating to 150 ° C. in a muffle furnace was used instead of the swing type induction heating furnace. The holding time was 0 minutes to 4 hours.

Next, for IHPNa of Examples 27 to 34, the amount of Na was measured from the battery capacity by the same method as that of IHPNa of Examples 8 to 25. FIG. 7 is a diagram showing the amount of Na in IHPNa of Examples 27 to 34. Note that FIG. 7 shows the amount of Na in combination with the IHP synthesis conditions used in Examples 27 to 34. In IHPNa of Examples 27 to 34, it was found that the amount of Na was in the range of 0 <a≤1.333. Further, as a result of crystal structure analysis of IHPNa of Examples 27 to 34 by the X-ray diffraction measurement method in the same manner as the crystal structure analysis of IHP of Examples 1 to 7, the crystals of IHPNa of Examples 27 to 34 The structure is tetragonal and belongs to the _{space group I4 1} / amd represented by the Hermann-Morgan symbol (international symbol).

(Battery performance evaluation test)
A coin cell was manufactured as a sodium ion battery, and the charge / discharge characteristics of the coin cell were evaluated. First, a method of manufacturing a coin cell will be described. Two types of coin cells were produced, one using a positive electrode active material made of IHP and the other using a positive electrode active material made of IHPNa. As the IHP, the IHP of Example 3 was used. As IHPNa, IHPNa of Example 18 was used. The size of the coin cell was φ20 mm in diameter × 3.2 mm in thickness.

First, the positive electrode of the coin cell will be described. A powdery positive electrode active material, a conductive material, a binder, and a thickener were mixed to prepare a slurry. Ketjen black was used as the conductive material. Styrene butadiene rubber was used as the binder. Carboxymethyl cellulose was used as the thickener. Regarding the mixing ratio of the positive electrode active material, the conductive material, the binder, and the thickener, the mass ratio was set to positive electrode active material: conductive material: binder: thickener = 80:10: 6: 4. .. Next, the slurry was applied to the surface of the aluminum foil and dried at 30 ° C to 120 ° C to form a positive electrode.

Metallic sodium was used for the negative electrode of the coin cell. As the electrolytic solution, _{a mixed solution of solute 1M NaClO 4} / solvent propylene carbonate and 2% by volume fluoroethylene carbonate was used. A non-woven fabric made of glass fiber was used as the separator.

Next, the charge / discharge characteristics of each coin cell were evaluated. The cut voltage was 1.5 V to 4.0 V, the test temperature was 25 ° C., the charge / discharge rate was 0.1 C, and the number of cycles was 3 cycles. FIG. 8 is a graph showing the charge / discharge characteristics of a coin cell using a positive electrode active material made of IHP. FIG. 9 is a graph showing the charge / discharge characteristics of a coin cell using a positive electrode active material made of IHPNa. In the graphs of FIGS. 8 and 9, the horizontal axis represents the capacity at the time of charging / discharging, the vertical axis represents the voltage, the case of charging is shown by a solid line, and the case of discharging is shown by a broken line. 8 and 9 both show the characteristics of the third cycle.

As shown in FIG. 8, the coin cell using the positive electrode active material made of IHP can be reversibly charged and discharged, and the capacity at the time of charging and discharging was about 150 mAh / g. In addition, no plate (flat part of potential) was observed during charging and discharging. As shown in FIG. 9, the coin cell using the positive electrode active material made of IHPNa can be reversibly charged and discharged, and the capacity at the time of charging and discharging was about 75 mAh / g. In addition, no plate (flat part of potential) was observed during charging and discharging. From this result, it was found that IHP and IHPNa have high performance as a positive electrode active material of a sodium ion battery. Further, when a positive electrode active material composed of IHP or IHPNa is used, since there is no plate (flat portion of potential) during charging / discharging, the potential required for sodium insertion changes continuously, so that the battery charge rate It was found that the estimation accuracy of was improved.

Next, the charge / discharge cycle characteristics of the coin cell using the positive electrode active material made of IHP of Example 3 were evaluated. The cut voltage was 1.5 V to 4.0 V, the test temperature was 25 ° C., the charge / discharge rate was 0.1 C, and the number of cycles was 100 times. FIG. 10 is a graph showing charge / discharge cycle characteristics. In the graph of FIG. 10, the horizontal axis represents the number of cycles, the vertical axis represents the discharge capacity, and the change in the discharge capacity is represented by a solid line. The initial discharge capacity is 110.6 (mAh / g), while the discharge capacity after 100 cycles is 71.8 (mAh / g) and the maintenance rate is 64.9%. rice field. From this result, it was clarified that the coin cell using the positive electrode active material made of IHP of Example 3 can be repeatedly charged and discharged even when the number of cycles is 100.

For the coin cell using the positive electrode active material made of IHP of Example 3, the structural change of the positive electrode active material during charging and discharging was evaluated. First, a method for evaluating structural changes will be described. There were four types of charge / discharge states: 0% discharge state (100% charge state), 50% discharge state, 70% discharge state, and 100% discharge state (0% charge state). The structural change of the positive electrode active material was evaluated by X-ray diffraction measurement. Rigaku's fully automatic horizontal multipurpose X-ray diffractometer SmartLab is used for X-ray diffraction measurement. The X-ray source is Cu, the acceleration voltage is 45 kV, the acceleration current is 200 mA, and the step angle is 0.02 degrees. The folding angle 2θ was measured under the condition of 20 ° to 30 °.

FIG. 11 is a graph showing the results of X-ray diffraction measurement of the positive electrode active material in each discharged state. The horizontal axis is the diffraction angle 2θ, the vertical axis is the intensity (arbitrary intensity), and the measurement data is shown by a solid line. In each discharge state, two peaks, a peak (a) and a peak (b), were observed. Further, the intensity of the peak (a) was higher than the intensity of the peak (b). More specifically, in the 0% discharged state (100% charged state), the peak (a) is 2θ = 26.55 ° or more and 26.95 ° or less, and the peak (b) is 2θ = 27.80 °. It was 28.20 ° or less. In the 100% discharged state (0% charged state), the peak (a) is 2θ = 26.05 ° or more and 26.45 ° or less, and the peak (b) is 2θ = 27.15 ° or more and 27.55 °. It was as follows. Further, the peak (a) and the peak (b) in the 50% discharged state and the 70% discharged state are the 0% discharged state (100% charged state) and the 100% discharged state (0% charged state). It was found that the peak (a) and the peak (b) were in the middle of each. From this result, the _{iron phosphate compound represented by Na a} Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d was measured by XRD measurement (X-ray diffraction measurement) using CuKα ray as an X-ray source. The first peak with the highest intensity in the range of 2θ = 26.05 ° or more and 26.95 ° or less, and the second highest intensity in the range of 2θ = 27.15 ° or more and 28.20 ° or less. It became clear that there was a peak of.

S10 Raw material solution preparation process S12 Hydrothermal synthesis process

Claims (16)

A positive electrode active material for sodium-ion batteries,
Na _a Fe _b (PO ₄ ) (OH) _c (H ₂ O) _d (However, 0 ≦ a ≦ 1.333, 1.000 ≦ b ≦ 1.333, 0 < c ≦ 1, 0 < d ≦ 1 look-containing iron phosphate compound represented by),
The iron phosphate compound satisfies c + d = 1 and
A positive electrode active material characterized in that the crystal structure of the iron phosphate compound _{belongs to the space group I4 1 / amd.} The positive electrode active material according to claim 1, wherein the positive electrode active material is used.
The iron phosphate compound has the highest intensity first peak in the range of 2θ = 26.05 ° or more and 26.95 ° or less in XRD measurement using CuKα ray as an X-ray source, and 2θ = 27.15. A positive electrode active material characterized by having a second peak with the second highest intensity in the range of ° or more and 28.20 ° or less. The positive electrode active material according to claim 1 or 2, wherein the positive electrode active material is used.
The iron phosphate compound is a positive electrode active material characterized by satisfying b-1 = c / 3 = (1-d) / 3. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 3. A sodium ion battery comprising the positive electrode according to claim 4. The method for producing a positive electrode active material according to any one of claims 1 to 3 for a sodium ion battery.
A raw material solution preparation process in which an iron raw material and a phosphorus raw material are dissolved in water to prepare a raw material solution, and
A hydrothermal synthesis step in which the raw material solution is heat-treated at 150 ° C. or higher and lower than 300 ° C. for hydrothermal synthesis.
A method for producing a positive electrode active material. The method for producing a positive electrode active material according to claim 6.
Wherein the iron raw material _{comprises Fe (NO 3) 3 · 9H} 2 O, FeC 2 O 4 · 2H 2 O, Fe at least one of _{(NH 4) 2 (SO 4} ) 2 · 4H 2 O,
A method for producing a positive electrode active material, wherein the phosphorus raw material contains H ₃ PO _4. The method for producing a positive electrode active material according to claim 6 or 7.
The raw material solution preparing step is a method for producing a positive electrode active material, which comprises containing a first sodium raw material in the raw material solution. The method for producing a positive electrode active material according to claim 8.
It said first sodium raw _material, and characterized in that it contains _{NaNO 3, Na 2 CO 3,} NaOH, CH 3 COONa, at least one of _{NaH 2 PO 4, Na 3 PO} 4 · 12H 2 O and _Na 2 HPO ₄ A method for producing a positive electrode active material. The method for producing a positive electrode active material according to claim 6 or 7.
The hydrothermally synthesized compound in the hydrothermal synthesis step and the second sodium raw material are mixed to form a mixture, and the mixture is heat-treated at 150 ° C. or higher and 500 ° C. or lower for a solid phase reaction. A method for producing a positive electrode active material. The method for producing a positive electrode active material according to claim 10.
Said second sodium raw _material, and characterized in that it contains _{NaNO 3, Na 2 CO 3,} NaOH, CH 3 COONa, at least one of _{NaH 2 PO 4, Na 3 PO} 4 · 12H 2 O and _Na 2 HPO ₄ A method for producing a positive electrode active material. The method for producing a positive electrode active material according to claim 11.
A method for producing a positive electrode active material, which comprises heat-treating the mixture in a reducing atmosphere to cause a solid-phase reaction. The method for producing a positive electrode active material according to claim 6 or 7.
A mixed solution is prepared by dissolving the compound hydrothermally synthesized in the hydrothermal synthesis step and the third sodium raw material in water, and the mixed solution is heat-treated at 150 ° C. or higher and lower than 300 ° C. for hydrothermal synthesis. A method for producing a positive electrode active material. The method for producing a positive electrode active material according to claim 13.
The third sodium raw _material, and characterized in that it contains _{NaNO 3, Na 2 CO 3,} NaOH, CH 3 COONa, at least one of _{NaH 2 PO 4, Na 3 PO} 4 · 12H 2 O and _Na 2 HPO ₄ A method for producing a positive electrode active material. The method for producing a positive electrode active material according to claim 13 or 14.
A method for producing a positive electrode active material, which comprises heat-treating the mixed solution in a reducing atmosphere for hydrothermal synthesis. The method for producing a positive electrode active material according to claim 6 or 7.
By assembling a battery and causing a discharge reaction by using the compound hydrothermally synthesized in the hydrothermal synthesis step as the positive electrode active material and the material containing sodium as the negative electrode active material, sodium is electrochemically added to the compound. A method for producing a positive electrode active material, which comprises inclusion.

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